1. Field of the Invention
The present invention relates to an electric machine and a power supply system having a battery pack for wirelessly transferring electric power through a coupling by a resonant magnetic field.
2. Description of the Related Art
An electric machine such as an electric vehicle is driven by an electric motor. For example, an electric vehicle runs by using an electric motor as the power source, as opposed to a car whose power source is an internal-combustion engine. An electric vehicle has a power battery installed therein, and gains driving force by transferring energy stored in a power battery to the electric motor. The power battery may be, for example, a secondary battery such as a lithium-ion battery, a nickel hydrogen battery or a lead battery. In many cases, a power battery is installed in a vehicle body in the form of a battery pack in which a plurality of “modules” are packaged together with a charge-discharge control circuit, etc., wherein each module includes a plurality of “cells” (the minimal form of a battery including an electrode and an electrolyte) connected together in series. When the remaining amount of electricity of the power battery becomes low, the battery pack is charged by being connected to an external power supply so that the electric vehicle can run again. The external power supply may be the commercial power supply (100 V/200 V), charging equipment installed in charging stations, etc., capable of high-power charging, etc.
The power battery of a conventional electric vehicle is charged at home or at a charging station (see, for example, Japanese Laid-Open Patent Publication No. 11-146504) each time the remaining amount of electricity of the power battery becomes low. FIG. 14 shows a power supply system for a conventional electric vehicle described in Japanese Laid-Open Patent Publication No. 11-146504. A power battery 72, which is a rechargeable secondary battery, is packaged together with a charge-discharge control circuit 61 into a battery pack 62. The battery pack 62 is secured inside the body of an electric vehicle 70 so that the battery pack 62 cannot be removed during normal use. The body of the electric vehicle 70 is provided with a vehicle-side connector 64 for receiving the electric power supplied from an external power supply 63. The battery pack 62 is provided with a battery pack-side charging connector 65, and a charging-side connector 65 and the vehicle-side connector 64 are connected together by a cable. The battery pack 62 is provided with a battery pack-side power output connector 66, and the battery pack-side power output connector 66 is connected to a power source 67 in the electric vehicle 70 by a cable. When charging, the user connects a power supply connector 69 that is provided at the end of a cable 68 of the external power supply 63 to the vehicle-side connector 64. The power battery 72 is charged by receiving the electric power from the external power supply 63 through the vehicle-side connector 64, the charging-side connector 65 and the charge-discharge control circuit 61.
Charging the power battery 72 takes some hours by normal charging, and some tens of minutes even by fast charging at higher voltages and currents. In addition to the large amount of time required for charging, there is another problem that the power battery 72 deteriorates when fast-charged repeatedly. In order to solve this problem, proposals have been made in which the power battery 72 whose battery level has become low is replaced with a fully-charged power battery (see, for example, Japanese Laid-Open Patent Publication No. 9-98518 and “2002 Report of research and study on car sharing systems using cars with replaceable battery” (March 2003, Mechanical Social Systems Foundation)).
In some fields other than electric machine such as electric vehicle, it has also been proposed to wirelessly charge a battery pack (see, for example, Japanese Laid-Open Patent Publication No. 10-248171). Japanese Laid-Open Patent Publication No. 10-248171 relates to a power supply device for use in a portable terminal device, in which electric power is transferred by an electromagnetic induction method. With this method, however, it is not possible to realize efficient transfer when there is a long distance between the power-transmitting antenna and the power-receiving antenna, or when the antennas are not well aligned with each other.
On the other hand, United States Patent Application Publication No. 2008/0278264 (FIGS. 12 and 14) discloses a new type of wireless energy transfer system for transferring energy from one of two resonators to the other, and vice versa, through the space between them. That wireless energy transfer system couples those two resonators with each other via the evanescent tail of the oscillation energy of the resonant frequency that is produced in the space surrounding those two resonators, thereby transferring the oscillation energy wirelessly (i.e., by a non-contact method).
With a conventional power supply system for an electric machine such as an electric vehicle, when an old battery pack is removed from the vehicle body to be replaced with another battery pack, it is necessary to unplug cables from the battery pack-side charging connector and the battery pack-side power output connector. When installing the new battery pack, it is necessary to plug the cables to these connecters. Moreover, it is necessary to establish an electric path by fitting together connecter terminals, and the connecter fitting operation requires a relatively large force. Moreover, as described in “2002 Report of research and study on car sharing systems using cars with replaceable battery” (March 2003, Mechanical Social Systems Foundation), it is necessary to provide measures to prevent electric shock during the operation or electric leak due to droplets such as rainwater.
On the other hand, with a technique for transmitting power to a household electric appliance by an electromagnetic induction method, it is not possible to realize efficient transfer when there is a long distance between the power-transmitting antenna and the power-receiving antenna, or when the antennas are not well aligned with each other.